1. Field of the Invention
The present invention relates to ultrasonic diagnostic imaging using Doppler shift measurement for detection and display of fluid flow velocities, and more particularly, to an imaging system that automatically calculates blood vessel size and flow velocity by use of a fuzzy logic technique.
2. Description of Related Art
Ultrasonic imaging techniques are commonly used to produce two-dimensional diagnostic images of internal features of an object, such as a human anatomy. A diagnostic ultrasonic imaging system for medical use forms images of internal tissues of a human body by electrically exciting an acoustic transducer element or an array of acoustic transducer elements to generate short ultrasonic pulses that travel into the body. The ultrasonic pulses produce echoes as they reflect off of body tissues that appear as discontinuities or impedance changes to the propagating ultrasonic pulses. These echoes return to the transducer, and are converted back into electrical signals that are amplified and decoded to produce a cross-sectional image of the tissues. These ultrasonic imaging systems are of significant importance to the medical field by providing physicians with real-time, high resolution images of the internal features of a human anatomy without resort to more invasive exploratory techniques, such as surgery.
The acoustic transducer which radiates the ultrasonic pulses typically comprises a piezoelectric element or matrix of piezoelectric elements. As known in the art, a piezoelectric element deforms upon application of an electrical signal to produce the ultrasonic pulses. In a similar manner, the received echoes cause the piezoelectric element to deform and generate the corresponding electrical signal. The acoustic transducer may be packaged within a handheld device that allows the physician substantial freedom to manipulate the transducer easily over a desired area of interest. The transducer would then be electrically connected via a cable to a central control device that generates and processes the electrical signals. In turn, the control device transmits the image information to a real-time viewing device, such as a video display terminal (VDT). The image information may also be stored to enable other physicians to view the diagnostic images at a later date.
One particular application of ultrasonic diagnostic imaging takes advantage of Doppler shift measurement to detect and display fluid flow velocities. In such a system, a region of interest within a patient is repetitively pulsed with ultrasonic signals, and the received echo signals are compared to a reference in order to determine a rate of flow of fluids through the region. The rate of flow can be determined from a measurement of the Doppler frequency shift of the received echo signals. As known in the art, the flow velocity can then be displayed within a colorized cross-sectional image in which different shading and color intensity represents flow rate and direction. These ultrasonic Doppler flow imaging systems are particularly useful in measuring volumetric blood flow through a vessel. In performing this type of circulatory system diagnosis, it is important to estimate the total volume of blood flow through the vessel rather than just the flow rate. If a portion of the artery were blocked due to stenosis or other such abnormal vascular condition, the blockage would appear as a substantial decrease in the blood flow volume.
Conventional Doppler flow imaging systems permit an operator to perform a volumetric blood flow estimation, albeit with a substantial level of manual intervention. The detected Doppler frequency shift is proportional to the blood velocity projection in the ultrasonic beam direction. In order to convert the measured speed to a volumetric flow measurement, it is necessary to first determine the angular difference between the direction of the ultrasonic beam and the direction of the vessel. Once the angular difference is known, the cross-sectional area of the vessel is estimated, and the volumetric flow rate determined by multiplying the estimated area with the mean velocity of the blood flow.
The angular difference is generally estimated by the ultrasound operator through visual interpretation of the image display. In a conventional ultrasound Doppler imaging system, the operator can manipulate a directional cursor on the image display to the center of a selected vessel in which a volumetric flow measurement is desired. The directional cursor is then rotated by the operator until its direction appears to coincide with the instantaneous linear direction of the vessel. Thereafter, the system calculates the angular difference based on the operator selected orientation of the directional cursor. Similarly, the cross-sectional area measurement of the vessel is based on an estimation of the vessel diameter by visually identifying the inner walls of the vessel on the image display. The operator then moves a pair of measurement cursors so that they appear to coincide with the inner vessel walls. The system then uses the distance between the measurement cursors to estimate the vessel inside diameter and cross-sectional area. From the estimated angular difference, estimated cross-sectional area, and mean velocity, the volumetric flow rate can be calculated.
Unfortunately, these visual estimation techniques are both unreliable and time consuming. The accuracy of the volumetric flow measurement is heavily dependent on the skill level of the operator. Even with a highly skilled operator, the directional and measurement cursors must be repositioned every time the transducer is moved to a new location. Any errors in the diameter estimation become magnified since the diameter is squared during the cross-sectional area calculation. To further complicate the diameter measurement, the vessel diameter may vary throughout a cardiac cycle. Finally, the existence of acoustic reverberations or other imaging artifacts that degrade image resolution tend to further increase the difficulty in visually identifying the orientation and position of the inner vessel walls.
Accordingly, a critical need exists for an ultrasonic diagnostic imaging system for detection and display of fluid flow velocities that is capable of accurately and automatically measuring the volumetric flow rate of a selected vessel. The imaging system should permit an operator to readily manipulate the transducer across a region of interest while obtaining an accurate volumetric flow measurement without having to continually reposition cursors on the display.